Medical implants and prostheses provide structural and mechanical aid or replacement for parts of the body that can no longer provide their intended function. Implants subject to stress must bear the required loads without failure, and must also be corrosion resistant and chemically compatible with body organs and fluids since they can remain in the body for years. Implants generally include metal wires, rods, plates, screws, tubes, and other devices. Some are attached to bone to allow others to reinforce damaged bone in the body. Since they are generally much stiffer than bone, they can promote stress shielding in the attached bone that can lead to implant loosening and osteoporosis. Implants generally have a lifetime of about 7-10 years. Surgical implant replacement is possible, however usually not more than twice for a particular implant device due to bone damage created by the first implant. As a result, recommended medical procedures involving implants are generally for older people.
Titanium alloys are usually materials of choice for making medical implants. In particular, Ti-6V-4Al, a titanium alloy initially developed for aerospace applications, is currently the alloy used to make most orthopedic implants and has been described in various papers and patents. For example, U.S. Pat. No. 4,854,496 by C. M. Bugle entitled xe2x80x9cPorous Metal Coated Implant and Method for Producing Same,xe2x80x9d which issued Aug. 9, 1989, describes an implant made by diffusion bonding titanium powder to a titanium or titanium Ti-6Al-4V alloy substrate. The coating provides the implant with enhanced biocompatibility. Additional examples of coated alloy implants now follow.
U.S. Pat. No. 5,725,573 entitled xe2x80x9cMedical Implants Made of Metal Alloys Bearing Cohesive Diamond Like Carbon Coatings,xe2x80x9d which issued Mar. 10, 1998, describes a titanium alloy implant with a diamond like coating.
U.S. Pat. No. 0,703,092 by D. D. Lee et al. entitled xe2x80x9cHydroxyapatite Coatings and a Method of their Manufacture,xe2x80x9d which issued Jun. 9, 1998, describes orthopedic and dental implants with a crystalline calcium phosphate coating. The coating anchors the implant to the existing bone and provides the implant with enhanced biocompatibility, which increases the useful life of the implant by minimizing the likelihood of implant rejection by the body.
U. S. Patent 5,817,326 by M. A. Nastasi entitled xe2x80x9cProcessing of Hydroxyapatite Coatings on Titanium Alloy Bone Prostheses,xe2x80x9d which issued Oct. 6, 1998, describes a method of bonding hydroxyapatite to titanium alloy substrates. A titanium alloy prosthesis is coated with a sol-gel of hydroxyapatite. After the coating hardens, the resulting coated prosthesis is subjected to ion implantation to increase the adhesion of the coating and the strength of the prostheses.
Although the Ti-6Al-4V alloy is generally considered to be chemically inert, biocompatible with human tissue, and corrosion resistant to human body fluids and other corrosive environments, vanadium and aluminum are potentially toxic. Normal wear leads to implant degradation and the release of alloy elements into the body. For example, vanadium has been observed in body tissues near Ti-6V-4Al alloy implants.
Potentially biocompatible titanium alloy substitutes for Ti-6Al-4V have been described in a recent paper by K. Wang entitled xe2x80x9cThe Use and Properties of Titanium and Titanium Alloys for Medical Applications in The USA,xe2x80x9d Mater. Sci. Eng., A213 (1996) 134-137. Screw implants of the titanium alloy Ti-15Mo-5Zr-3Al (TAMZ) are described in a paper by A. Ungersbock, S. M. Perren, and O. Pohler entitled xe2x80x9cComparison of tissue reaction to implants of a beta titanium alloy and pure titanium. Experimental study on rabbits,xe2x80x9d J. Mater. Sci.: Materials in Medicine, 5 (1994) 788-792. Although these implants do not contain vanadium and showed promise after testing in rabbits over a three-month period, they still contain aluminum. U.S. Pat. No. 5,782,910 by J. A. Davidson entitled xe2x80x9cCardiovascular Implants of Enhanced Biocompatibility,xe2x80x9d which issued Jul. 21, 1998 describes medical implants made from low-modulus Tixe2x80x94Nbxe2x80x94Zr alloys. The implants can be surface hardened with a hard, wear-resistant, hemocompatible ceramic material.
A more benign replacement for titanium alloy implants may solve the problem of the release of toxic elements into the body from degraded alloy implants. An implant of pure titanium could be the ideal replacement since it is lightweight, chemically and biologically more compatible with human tissue, and can rigidly fixate to bone better than a titanium alloy implant. Unfortunately, pure titanium lacks sufficient strength for general use as an implant material; while Ti-6Al-4V alloy has a yield strength of about 795 MPa and an ultimate strength of 860 MPa, the yield strength and ultimate strength for pure titanium are only about 380 MPa and 460 MPa, respectively.
Clearly, strong, lightweight, corrosion resistant implants that are chemically and biologically compatible with human fluids and tissue are highly desirable. Therefore, an object of the present invention is a strong, lightweight, corrosion resistant material that is chemically and biologically compatible with human fluids and tissue.
Another object of the invention is a strong, lightweight, corrosion resistant medical implant that is chemically and biologically compatible with human fluids and tissue.
Yet another object of the present invention is a method of providing ultrafine-grained titanium of sufficiently high strength for general use in medical implants and other devices.
Still another object of the present invention is a titanium implant having strength equal to or exceeding that of Ti-6Al-4V.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as embodied and broadly described herein, the present invention includes medical implants made from ultrafine-grained titanium. A coarse-grained titanium billet is placed into a lubricated first channel of a preheated equal channel angular extrusion die. The die has a second channel equal in diameter or slightly narrower than the first channel. The warmed billet is extruded through the second channel of the die, rotated about its longitudinal axis, and extruded again. After repeatedly rotating and extruding the billet, it is subjected to cold rolling and/or cold extrusion to provide ultrafine-grained titanium that is used to make medical implants.